Help, my system seem to have slowly gotten " bright"

My 1st thoughts were it has something to do with the speakers. Did you move or adjust your speakers recently? Move any of the surrounding furniture? I recently had issues with fatigue after messing with my speaker placement. That was an eye opening experience for me. 

As far as equipment goes since you ruled out the amp/preamp I would wonder if the power supply of your source is going bad. Or perhaps some other capacitors as others have mentioned. 

On a side note..does adding more bass actually make a speaker sound less "bright" or fatiguing? 

Obviously my thoughts on what could be causing your sudden "brightness" are just guesses. Please share when/if you figure out what the cause is. 

If you didn't make any changes to the gear, placement, etc. maybe it's your hearing.

you have gotten older get your hearing checked before you do anything you might regret

@rhg3 has a great suggestion.  Listen to one frequency at a time.  If you have a way to play youtube through your system, test frequencies are available on youtube.

As for hearing suggestions, your change is in the opposite direction of normal hearing degradation.  You lose high frequencies as you get older.  Many expensive speakers teen to be a bit bright because the target market is older people with losses in the highs.  I guess it isn't impossible for hearing loss to be in the low frequencies.

As for my suggestion on bass, realize that you will turn it up until the bass sounds right.  If you have lost bass, then you will turn up the volume until the bass sounds right and you will have turned up the high frequencies so high they seem bright.  It isn't whether you have decent bass, it is whether you have a balance between bass and treble.

Jerry